1. Field of the Invention
This invention relates to conveyors having a series of rollers individually driven from a common source, to support and transport articles such as printed circuit cards.
2. Description of the Prior Art
The prior art, as exemplified by U.S. Pat. Nos. 2,856,741, 3,078,024 and 3,360,260, discloses conveyors having a series of rollers adapted to convey a particular article, the rollers typically being interconnected by an endless chain or by gears driven from a common source. In many applications, chain or gear drives as provided by the prior art may be objectionable on the basis of initial cost, since a substantial number of relatively expensive sprockets and gears are generally required. These drives may further be objected to for their noise of operation, danger to personnel, and their costs of maintenance. The objectionable features of gear and chain drives can be avoided by using a friction drive belt engaging each roller; however, prior belt driven devices have typically provided an idler pulley between adjacent rollers to cause the belt to flex and follow an arc around part of the circumference of each roller, belt tension being relied on to maintain sufficient normal force between the belt and each roller to avoid slippage. Since belt tension varies with changes in belt length produced by wear and stretching, means must be provided to compensate for changes in belt length, such as biasing the idler pulleys between adjacent rollers laterally against the belt. It has been found, however, that this arrangement of conveyor rollers and biased pulleys may produce undesirably high belt tensions, and that flexing of the belt between rollers reduces belt life.